Electronic musical instrument employing differential transformer for signal coupling

ABSTRACT

An electronic musical instrument in which electronically generated tones are coupled from a generating system to a power amplifier by a differential transformer in which primary and secondary windings are normally disposed transversely to each other on respective open cores which are normally disposed in parallel, proximate relationship to effect null transfer of signal, and which are moved with respect to each other to effect desired transfer of signal.

United States Patent Hopping [4 1 Jan. 25, 1972 [54] ELECTRONIC MUSICAL INSTRUMENT 21:32:, uorlalping ..84/l.16 I a er ...84/D1G. 7 3,353,030 1 H1967 Michel ..84/D1G. 7 COUPLING FOREIGN PATENTS OR APPLICATIONS [72] Inventor; Alvin S, Hopping, Nolans Point, Lake 838,670 3/1939 France ..84/1.26 Hopatcong, NJ. 07849 1,138,995 10/1962 Germany ...84/1.09 478,142 1/1938 Great Britain ..84/l.26 [22] Filed: Apr. 15, 1970 [2|] APPL 23,570 Primary Examiner-Thomas J. Kozma Assistant Examiner-Stanley J. Witkowski Attorney-Herbert Smith Sylvester [52] US. Cl. ..84/1.15, 84/D1G. 7 [51] Int. Cl ..Gl0c 3/20 57 ABSTRACT [58] Field ofSearch ..84/D1G.7, 1.09, 1.1, 1.12-1.15,

84/ 1.26, 1,27 n electromc musical instrument in WhlCh electronically generated tones are coupled from a generating system to a 56] R f Cited power amplifier by a differential transformer in which primary and secondary windings are normally disposed transversely to UNITED STATES PATENTS each other on respective open cores which are normally 1 disposed in parallel, proximate relationship to effect null :g RMamn Q transfer of signal, and which are moved with respect to each $1965 233;; 4 26 other to effect desired transfer of signal.

7 I U 1 "'7 v 3 Claims, 6 Drawing Figures OSCILLATORS volcmc; S Q Z Q' lllll'llhllilln 1| 1 amm PATENTED JAN25 I972 POWER AMPLIFIER INVENTOR. ALVIN S. Hoppme ELECTRONIC MUSICAL INSTRUMENT EMPLOYING DIFFERENTIAL TRANSFORMER FOR SIGNAL COUPLING This invention relates to musical instruments in which electronic musical tones are modified to confer upon them desired attack and decay and harmonic-content characteristics.

In the'operation of electronic musical instruments it is not satisfactory simply to turn the tones on and off using a simple On-Off contact switch because the use of such a switch results in an unpleasant click. This click arises from the gradual buildup of sound (attack) experienced in conventional musical instruments (which are also characterized by a gradual reduction of sound from a maximum to zero (decay) when tone generation stops).

In the starting and termination of each note by a musician, it is desirable that each note start and end gradually without clicks, thumps or self-generated noise, and that this operation be controlled by the musician e.g., by digital pressure. In addition, it should be possible to control tones by minimal digital pressure so that playing speed is not unduly restricted and fatigue does not become an important factor. Finally, it is desirable that the operation be characterized by a reasonably high efficiency so that the signal to noise ratio of the device does not suffer unduly and that sufficient drive is available to operate the balance of the musical instrument.

Various methods have been proposed in the past for keying of electronic musical instruments to produce appropriate attack and decay characteristics and gradual volume control. A

variable resistance controlled by the appropriate tone selecting device is one method which has been proposed.

The use of a variable resistance as a control device offers a high degree of efficiency of energy transfer in that substantially I percent (or substantially 0 percent) of the energy can readily be transferred by this means, however, it suffers from the drawback of generating noise and in the conventional potentiometer type of embodiment, requires rotation of from 180 to 270, which leads to mechanical complexities and a requirement for excessive energy of rotation.

The apparatus of the present invention is characterized by substantially complete absence of internally generated noise, an energy transfer efficiency typically of over 90 percent (or less than 1 percent), mechanical rotation or physical displacement on the order of only to and a requirement of a very low energy to accomplish such rotation. The instant device readily couples notes of all frequencies in the audio spectrum with equal freedom from clicks, thumps, and selfgenerated noise, over a broad range of four or more octaves. In practice, it is quite feasible for the instant device to be characterized by a null or zero point so hear zero (/Adb) as to be substantially inaudible while utilizing a moving part weighing under 0.25 ounces, thereby insuring speed and ease of operation without incurring operator fatigue.

The invention will be further described in connection with the accompanying drawings which are to be considered as exemplary of the invention and do not constitute limitation thereof.

In the drawings:

FIG. 1 illustrates a view, in plan, of an electronic musical iristrument in accordance with the invention in which the instant coupling device is illustrated structurally and the electronic components are shown in block diagram form;

FIG. 2 is a view, in elevation, of the structural elements of FIG. I;

FIG. 3 illustrates, in detail, the pole pieces and core piece used in the primary of the transfonner shown in FIGS. I and 2;

FIG. 4 shows, in detail, the pole pieces and core piece used in the secondary of the transformer illustrated in FIGS. 1 and FIG. 5 is a view, in elevation, of

FIG. 6 is a view, in elevation, of the coupling transformer of FIG. 5. 5.

The present invention is illustrated in use on an electronic musical instrument of the nature of that shown in my U.S. Pat.

Nos. 3,223,77l and 3,322,877, in which a musical instrument is provided with bars 10 associated with the strings of a conventional four-stringed instrument, i.e., an E-string, an A- string, a D-string and a G-string. In the course of playing the instrument, the bars 10 are depressed to accomplish note selection and to actuate the appropriate tone-generating oscillators 20. (Reference is made to my previously mentioned U.S. Pat. for details of accomplishing such.)

The tones so generated are conducted (in the form of elec- 0 trical signals, of course) of course) by leads 21, 22 to voicing circuits 25. In these voicing circuits the harmonic content of the previously generated tones may be enriched or depleted, subharmonics may be added, or other effects may be accomplished in a known manner to adjust, as desired, the characteristics of the signal.

The signal, modified as desired by the voicing system 25, is then delivered by leads 26 and 27 to a generally rectangular primary winding 30 of a differential transformer, the latter being indicated generally by the reference character 33.

As shown in the Figures, the primary winding 28 of FIGS. 1-4 constitutes a generally rectangular winding which is disposed in a horizontal plane. A pair of pole pieces 35 and 37 project laterally beyond the side of the coil 30 along the top and bottom ends thereof and abut a single pole piece 29 in the center of the core. Suitably, those pole pieces and core piece may be made of 0.0l4-inch thick transformer steel lamella and may be only one-half inch long, a coil height of about onefourth inch being suitable.

The primary winding, core and pole pieces form an assembly 39 which is supported about a transverse axis by means of a shaft 40 joumaled in pivots 42 in a pair of mounting posts 44 which in turn project upwardly from a base 48.

The bars or keys 10 are each supported by means of an adjustable screw 50 resting on a platform 52 which is affixed to the under side of the primary assembly 39. A resilient spring 55 serves to counterbalance the downward pressure of the bars 10 against the platform 52, and to bias the primary assembly 39 to a constant position from which the primary assembly may be tilted by downward pressure on any of the keys 10 by the application of digital pressure thereto.

A fixed, stationary secondary assembly 59 is disposed with its generally rectangular winding 60 in a generally vertical plane at 90 to the primary winding 30 on a pair of U-shaped pole pieces 65 and 67 having a core piece 69 therebetween. The open end of the U-shaped pole pieces 65 and 67 oppose the open end of the U-shaped core formed by the pole piece 35 and 37 of the primary assembly.

The secondary pole pieces 65 and 67 and core piece 69 are each about one-half inch long and one-half inch wide and also are suitably fabricated from 0.0l4-inch thick transformer steel. The entire secondary assembly is affixed to a spacer block 75 carried on one end of a cantilever 77, the other end thereof being affixed to the base 48 by a bolt and washer 81. An adjusting screw is provided to facilitate biasing of the primary and secondary with respect to each other so that minimum signal is transferred in the normal, rest position.

In operation, digital pressure on any of the keys or bars 10 serves to rotate the primary assembly 39 on its pivots 42 thus tilting the primary from its normal null or balanced position in which substantially less than 1 percent energy transfer occurs between the primary and the secondary to accomplish the desired transfer of energy, depending upon the degree of relocation. Typically, within a 10 displacement of the primary assembly, an energy transformer of over percent may be accomplished.

The embodiment illustrated in FIGS. 5 and 6 is similar to that of FIGS. 1-4 with the modification that a generally circular winding is used for the primary, in lieu of the rectangular winding illustrated in FIGS. 1-4, and the secondary has a core 169 but no pole pieces. This modified version, although effective, is not as efficient as that illustrated in FIGS. 1-4 and therefore the embodiment of FIGS. 1-4 is preferred. For example, using a comparable components and conditions, a

spread of 64 db between maximum signal transfer and minimum signal transfer is feasible with the embodiments of FIGS. 1-4 whereas with the embodiment of FIGS. 5 and 6, a spread of on the order of only 32 db can be achieved.

Referring again to FIGS. 1-4, the signal coupled to the secondary 60 is directly transmitted therefrom to the power amplifier by leads 85, 86 from which it is directed to the output speaker 90. In this manner, i.e., the disposition of the coupling transformer 33 subsequent to all voicing and switching operations, the latter are completely inaudible in the output of the musical instrument to the extent they are completed while the transformer is in its null position, which is normally the case. On the other hand, the instant coupling system is substantially noise-free and stable, and after initial adjustment of the zero adjusting screw 85, remains stable without any evidence of drift, noise generation, effects of wear or the like.

It is to be understood that the invention herein illustrated and described is to be limited only by the scope of the appended claims and that various changes may be made in details of construction without departing from the true spirit of the invention.

What is claimed is:

1. An electronic musical instrument which comprises an electronic tone generator, a power amplifier for amplifying said tones, a transducer for converting the output of said power amplifier into audible sound, and a differential transformer coupling said tones between said tone generator and said power amplifier, said transformer having a ferromagnetic primary core having a base pole piece and two outside leg pole pieces assembled in U-shaped cross section, a rectangular primary winding wound about said base pole piece and between said outside pole pieces, said outside leg pole pieces projecting laterally beyond the periphery of said rectangular coil, a mount in which said U-shaped primary core is pivotally journaled means to pivot the open end of said U-shaped primary core about the base pole piece thereof, means for biasing said primary core to a normal rest position, a ferromagnetic secondary core having a pole piece fixedly disposed in the plane of normal rest of said rectangular primary winding and in proximity to the open end of said U-shaped primary core, a rectangular secondary winding wound about said secondary pole piece in a plane substantially perpendicular to the plane of normal rest of said primary winding, and means to adjust said primary and secondary with respect to each other to transfer minimum signal in said normal rest position, said windings being normally biased to a rest position of less than 1 percent transfer of energy from said primary winding to said secondary winding and said primary being pivotable to a position of at least percent transfer of energy on rotation of not more than 10.

2. An electronic musical instrument as set forth in claim 1, in which said secondary pole piece is a projecting middle leg of a three-leg, E-shaped core, the ends of the legs of said U- shaped primary core and of said E-shaped secondary core being disposed in opposed, interleaved proximity to each other in said normal rest position.

3. An electronic musical instrument which comprises an electronic tone generator, a power amplifier for amplifying said tones, a transducer for converting the output of said power amplifier into audible sound, and a-differential transfon'ner coupling said tones between said tone generator and said power amplifier, said differential transformer having a primary winding and a secondary winding, one of said windings being on a first ferromagnetic core with open pole pieces and the other winding being on a second ferromagnetic core, means for disposing and biasing said windings substantially transversely with respect to each other and said open pole pieces and second ferromagnetic core normally in substantially parallel, end-to-end proximity in which minimum net signal is inductively transferred between said windings, and means for moving said open pole pieces with respect to said second ferromagnetic core to effect a substantial net inductive transfer of signal between aid windings. 

1. An electronic musical instrument which comprises an electronic tone generator, a power amplifier for amplifying said tones, a transducer for converting the output of said power amplifier into audible sound, and a differential transformer coupling said tones between said tone generator and said power amplifier, said transformer having a ferromagnetic primary core having a base pole piece and two outside leg pole pieces assembled in U-shaped cross section, a rectangular primary winding wound about said base pole piece and between said outside pole pieces, said outside leg pole pieces projecting laterally beyond the periphery of said rectangular coil, a mount in which said U-shaped primary core is pivotally journaled means to pivot the open end of said U-shaped primary core about the base pole piece thereof, means for biasing said primary core to a normal rest position, a ferromagnetic secondary core having a pole piece fixedly disposed in the plane of normal rest of said rectangular primary winding and in proximity to the open end of said U-shaped primary core, a rectangular secondAry winding wound about said secondary pole piece in a plane substantially perpendicular to the plane of normal rest of said primary winding, and means to adjust said primary and secondary with respect to each other to transfer minimum signal in said normal rest position, said windings being normally biased to a rest position of less than 1 percent transfer of energy from said primary winding to said secondary winding and said primary being pivotable to a position of at least 90 percent transfer of energy on rotation of not more than 10*.
 2. An electronic musical instrument as set forth in claim 1, in which said secondary pole piece is a projecting middle leg of a three-leg, E-shaped core, the ends of the legs of said U-shaped primary core and of said E-shaped secondary core being disposed in opposed, interleaved proximity to each other in said normal rest position.
 3. An electronic musical instrument which comprises an electronic tone generator, a power amplifier for amplifying said tones, a transducer for converting the output of said power amplifier into audible sound, and a differential transformer coupling said tones between said tone generator and said power amplifier, said differential transformer having a primary winding and a secondary winding, one of said windings being on a first ferromagnetic core with open pole pieces and the other winding being on a second ferromagnetic core, means for disposing and biasing said windings substantially transversely with respect to each other and said open pole pieces and second ferromagnetic core normally in substantially parallel, end-to-end proximity in which minimum net signal is inductively transferred between said windings, and means for moving said open pole pieces with respect to said second ferromagnetic core to effect a substantial net inductive transfer of signal between said windings. 